When drilling wells pipe frequently becomes stuck in the well, which hinders further drilling operations. This will stop the drilling. To continue drilling, the drill string needs to be freed or removed. If this cannot be done, the well must be abandoned and a new well started. Thus, the most economical approach is to remove the free drill pipe and loosen the stuck pipe so that it can be removed, and a new drill pipe inserted. Thus, it is desirable to be able to ascertain as near as possible the location where the drill pipe is stuck so that the free pipe above the stuck portion can be recovered and the stuck portion can be loosened and recovered. Free point tools are used to locate the "free point" that point in the pipe string just above where the pipe is stuck. The stuck pipe can either be casing pipe or tubing pipe.
In the past a number of apparatus have been developed for determining free point. These devices usually located the free point by determining stresses in the pipe which would indicate whether the pipe was free or was stuck at certain locations. Many of these prior tools required two trips or more down the well in order to make an accurate determination of the free point.
Also, it is common in free point operations to attach to the free point tool an explosive charge called a string shot. This charge is positioned across the lowest free collar. The drill pipe is torqued with left hand torque and the string shot fired. After the pipe has been backed off by the explosive charges, the free pipe can be removed from the well. Thereafter, typically washing operations are conducted to free up the previously stuck pipe and allow its removal. Detonation of these explosive charges creates a great deal of stress on the free point tool. Previous free point tools normally contained oils and were pressurized to achieve a pressure balance. This use of oils in the tools created maintenance headaches, potential for leaks and possible contamination. Further, many times the tool apparatus was damaged or destroyed by detonation of the cutting charge (string shot) suspended below the device. Alternatively, the tool had to be recovered and the explosive charge sent down separately, which not only required an additional trip down the well bore, but also was subject to improper placement of the charge. Some prior tools could not independently measure torque and stretch. Many had the limitation that they could only take torque measurements in one direction.
A free point tool should be small so that it can pass through the special parts of the drill stem that has reduced internal diameter. The tool should be able to withstand high temperatures and work when in a non-vertical position. The tool should be easily transportable by helicopter. The tool should also be tough enough to survive the shock of detonation of a string shot. Prior to the present invention, free point tools were constructed in two parts with sensors mounted in between. These designs are oil filled to balance pressure. Thus, damage to the sensors and oil seals frequently resulted in damage from high temperature and also by absorbing the shock of the string shot.
Thus, there has been a continuing need to provide an improved free point locating tool. The present invention is advantageous in that it eliminates many of the moving parts of previous tools, simplifies construction, does not require use of oil or other fluid to achieve pressure balance so that the tool will operate. The tool of the present invention is also easily assembled and disassembled for transportation to the job site, is less prone to damage caused by detonation of an explosive charge suspended below the tool, and can be made in smaller diameters than are possible with current tool design.
The free point tool of the present invention has the advantages:
(a) it can make independent measurement of torque and stretch; PA1 (b) it can read both left and right hand torque; PA1 (c) it has good resolution; PA1 (d) it has a linear signal and a wide range; PA1 (e) it is relatively free of error induced by temperature; PA1 (f) it is rugged and able to withstand repeated back off shots; PA1 (g) it can be disassembled for transport; PA1 (h) the tool as assembled is essentially a one piece main body with a movable sensor sleeve over a portion of the main body; PA1 (i) it has a sensor sleeve that requires very little force to operate; and PA1 (j) it has a construction that will allow a lot of weight to be suspended on the bottom of the tool without inducing a measurement error.